Inter-module battery balancing using voltages to select battery sub-modules to power loads

ABSTRACT

One or more battery sub-modules are selected by obtaining at least one voltage from each battery sub-module and selecting based at least in part on the obtained voltages. The battery sub-modules are electrically connected in series in order to provide power to a primary load. Each battery sub-module includes a plurality of cells electrically connected in series and each battery sub-module further includes a battery management system that monitors the cells in that battery sub-module. Those battery management systems in the selected sub-modules are turned off so that the battery management systems in the selected sub-modules do not consume power at least temporarily from the cells in the selected sub-modules while (1) the battery sub-modules are not providing power to the primary load and (2) the battery sub-modules are not being charged.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/046,312 entitled INTER-MODULE BATTERY BALANCING USINGMINIMUM CELL VOLTAGES TO SELECT BATTERY SUB-MODULES TO POWER LOADS filedJul. 26, 2018 which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

New types of aircraft that are all-electric are being developed. Due todifferences in how the batteries tend to be designed (e.g., batteries inaircraft have to satisfy the Federal Aviation Administration, which mayhave more concerns about single points of failure and degrees ofredundancy compared to the National Highway Traffic SafetyAdministration) and/or how the vehicles are used, there may be somebattery-related issues which are exposed as all-electric aircraft aredeveloped which were not previously exposed with electric cars. Newtechniques to detect, mitigate, and/or avoid such battery-related issuesin all-electric aircraft (or other vehicles) would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a flowchart illustrating an embodiment of a process forinter-module balancing.

FIG. 2 is a diagram illustrating an embodiment of a battery system whichincludes battery sub-modules connected together in series where eachbattery sub-module includes cells connected together in series.

FIG. 3A is a diagram illustrating an embodiment of a battery sub-modulewithout its lid on.

FIG. 3B is a diagram illustrating an embodiment of a battery sub-modulewithout its lid on.

FIG. 4 is a flowchart illustrating an embodiment of a process forinter-module balancing, including by turning off electronics.

FIG. 5 is a flowchart illustrating an embodiment of a process forinter-module balancing, including by configuring a set of electronics todraw power from an unselected battery sub-module.

FIG. 6 is a diagram illustrating an embodiment of cell voltages inbattery sub-modules in a battery system.

FIG. 7 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold.

FIG. 8 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold and maximums of minimumcell voltages.

FIG. 9 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold and maximums of maximumcell voltages.

FIG. 10A is a diagram illustrating an embodiment where balancing isperformed both before and after charging.

FIG. 10B is a diagram illustrating an embodiment where balancing isperformed only after charging.

FIG. 11 is a flowchart illustrating an embodiment of a process to decidewhen to perform balancing relative to a charging process.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a technique to balance battery sub-modules in abattery system are described herein. In some embodiments, balancing isperformed by receiving, for each battery sub-module in a plurality ofbattery sub-modules (e.g., connected together in series), a voltageassociated with a cell in that battery sub-module, where each batterysub-module in the plurality of battery sub-modules includes a pluralityof cells (e.g., connected together in series). A battery sub-module isselected from the plurality of battery sub-modules based at least inpart on the received voltages. A set of one or more loads (e.g.,electronics or other consumers of power), which draws power from theselected battery sub-module and is not powered by any other batterysub-module in the plurality of battery sub-modules, is configured sothat the set of one or more loads at least temporarily does not drawpower from the selected battery sub-module.

In some applications, this technique is used to select which batterysub-modules do not (e.g., at least temporarily) supply standby orvampire power to corresponding electronics while a primary load (e.g.,the lift fans in an all-electric aircraft) is not drawing power from thebattery system. In the long run, if this process is performed, then thebattery sub-modules will be more balanced than if the process had notbeen performed (e.g., where battery sub-modules which are more balancedare better for performance) and/or permanent damage to the batterysub-modules can be avoided.

FIG. 1 is a flowchart illustrating an embodiment of a process forinter-module balancing. In some embodiments, the process is performed byand/or on a battery system with multiple sub-modules connected togetherin series where each battery sub-module in turn includes multiple cellsconnected together in series.

At 100, for each battery sub-module in a plurality of batterysub-modules, a voltage associated with a cell in that battery sub-moduleis received, wherein each battery sub-module in the plurality of batterysub-modules includes a plurality of cells. In one example, a batterysystem is used to power an all-electric aircraft. For a variety ofreasons, the battery system which powers the aircraft may be made up ofmultiple battery sub-modules which are connected together in series. Forexample, by having multiple battery sub-modules connected together inseries to form the overall battery system, the battery sub-modules canbe easily replaced as or if needed, and relatively high voltages (e.g.,on the order of hundreds of volts, which is required by the lift fans)as well as lower voltages (e.g., on the order of single volts, which isrequired by the avionics and/or electronics) are simultaneouslyavailable. In contrast, these desirable characteristics and/or featuresare not present when the battery system comprises a (more) monolithicbattery. An exemplary battery system, which includes battery sub-modules(connected together in series) which in turn include cells (alsoconnected together in series) is described in more detail below.

At 102, a battery sub-module is selected from the plurality of batterysub-modules based at least in part on the received voltages. Forexample, the selected battery sub-module may have been selected becauseit is undesirable for that battery sub-module to continue supplyingpower to one or more loads (e.g., electronics, motors, solenoids, etc.)which are running off of the selected battery sub-module. In someembodiments, more than one battery sub-module is selected. Some examplesof how the selection may be performed are described in more detailbelow.

At 104, a set of one or more loads, which draws power from the selectedbattery sub-module and is not powered by any other battery sub-module inthe plurality of battery sub-modules, is configured so that the set ofone or more loads at least temporarily does not draw power from theselected battery sub-module. As will be described in more detail below,in some embodiments, the loads comprise electronic(s) which are turnedoff so that they no longer draw (e.g., vampire) power from the selectedand/or associated battery sub-module. Alternatively, the electronic(s)associated with the selected battery sub-module may be configured sothat they draw power from some other battery sub-module other than theselected one.

Conceptually and/or generally speaking, the above process attempts tobalance the voltage levels in the various sub-modules and/or cells byselectively permitting some (but not all) battery sub-modules to providepower to associated loads, for example during some quiescent or restingstate when an aircraft (or other load) is not consuming substantialamounts of power. This draws down the voltage levels in sub-modulesand/or cells (e.g., which are better equipped and/or in a better stateto provide power) so that voltage levels in the selected sub-modulesand/or selected cells can be preserved.

In one example of why balancing is important and/or useful, if nobalancing is performed (e.g., per the process of FIG. 1), then somebattery sub-modules in the battery system will permanently fail if thebattery system is left alone for ˜20 days. If the battery system is usedin an aircraft, this is entirely possible. For example, the pilot mayfly the aircraft somewhere remote with no charging station where theaircraft sits idle for ˜20 days and the battery not being charged duringthat time. Or, the aircraft could be left in a hangar for long periodsof time so maintenance requiring human intervention would be quiteinconvenient.

Another benefit to keeping the battery sub-modules balanced is that itincreases the capacity of the battery due to the fact that for batteriescomprising sub-modules in series, the battery's capacity is driven bythe minimum capacity cell. This is because discharging a battery belowits minimum capacity will damage it. In the same vein, keeping thesub-modules balanced decreases the time it takes to charge since in thebalanced state, the cells will be at a uniform and higher voltagerelative to the imbalanced state. Finally, maintaining a balancedbattery can increase its overall life. Cells with depressed voltages maydegrade more quickly than their neighbors and the sub-module must bereplaced when one of its component cells reaches a critical point ofdegradation. Furthermore, in embodiments that allow for sub-modules tobe discharged in parallel, sub-modules at different voltages willcontribute different currents its load and sub-modules that mustcontribute excess current will experience accelerated degradation. Theprocess of FIG. 1 may be repeated as or if desired. For example, withthe unselected battery sub-modules providing power, the voltage levelsstored in those cells and/or battery sub-modules will go down, resultingin different voltage levels and thus different degrees and/or states ofimbalance. In one example, the battery sub-modules selected at step 102do not supply power during step 104 for 15 minutes (as an example),after which the process of FIG. 1 is repeated with updated voltages. Asa result, different battery sub-modules may be selected at step 102 toat least temporarily no longer supply power to their correspondingelectronics.

In some embodiments, the exemplary balancing process described above isperformed when a primary load (e.g., the lift fans in an all-electricaircraft) is not drawing power. For example, the draw by the primaryload on the battery system may change very quickly and so it may bedifficult and/or expensive to sample the battery system sufficientlyfast enough to accurately determine what state the battery system is inwhen the primary load is drawing power. For this reason, it may besimpler and/or easier to perform balancing when the primary load is off.

In some embodiments, the exemplary balancing process described above isperformed before and/or after charging of the battery system isperformed. For example, by balancing the battery system (e.g., per theprocess of FIG. 1) before charging of said battery system occurs mayhelp with the charging process itself by fixing or otherwise reducinglarge(r) imbalances (if any) between battery sub-modules (e.g., whichmay be undesirable during the charging process). If balancing (e.g., perthe process of FIG. 1) is performed after the battery system is charged,then small(er) imbalances between the battery sub-modules may be fixedor otherwise reduced.

It may be helpful to describe an exemplary battery system which performsthe process of FIG. 1. The following figure describes one such exemplarybattery system.

FIG. 2 is a diagram illustrating an embodiment of a battery system whichincludes battery sub-modules connected together in series where eachbattery sub-module includes cells connected together in series. In thisexample, the battery system is used to power an all-electric aircraft.

In this example there are M battery sub-modules: a first batterysub-module (200 a), a second battery sub-module (200 b), and an M^(th)battery sub-module (200 c) where the battery sub-modules are connectedtogether in series. This produces a high voltage power source (e.g., onthe order of hundreds of volts) which powers a high-voltage load (202),such as the lift fans of the aircraft.

Each battery sub-module, in turn, includes N cells which are connectedtogether in series. For example, the first battery sub-module (200 a)includes a first cell (204 a), a second cell (204 b), an (N−1)^(th) cell(204 c), and an N^(th) cell (204 d). The voltage across each batterysub-module in this example is on the order of tens of volts. In thisexample, there are 36 battery sub-modules and 12 cells per batterysub-module. The following figures show an exemplary battery sub-module.

FIG. 3A is a diagram illustrating an embodiment of a battery sub-modulewithout its lid on. In the example shown, the battery sub-moduleincludes layers of cells (300) interleaved with layers of (e.g.,fire-retardant) insulation (302). In this example, the cells are pouchcells which perform better when pressure is applied (e.g., ˜3-5 PSI).More specifically, the cycle life of pouch cells can be extended byapplying pressure to the pouch cells. As such, the battery sub-module isencased by a metal can (304) which applies pressure on the containedpouch cells.

Each of the cells has two tabs (306) which extend upward from the cell:a positive tab and a negative tab. The tabs are connected together sothat the cells are connected together electrically in series. See, forexample FIG. 2.

FIG. 3B is a diagram illustrating an embodiment of a battery sub-modulewithout its lid on. In this example, a lid (350) has been attached tothe battery sub-module, so that only a single positive connection and asingle negative connection are exposed. In the example described abovewhere the battery system is included in an aircraft, each batterysub-module may be physically and electrically connected together withinthe aircraft so that a single battery sub-module can be swapped out andreplaced as or if needed.

Returning to FIG. 2, each battery sub-module (200 a-200 c) has a set ofelectronics (206 a-206 c) which are associated with it and are poweredby that battery sub-module (e.g., even when the aircraft is not flyingand the high-voltage load (202) is not consuming power). For example,the first set of electronics (206 a) is powered by the first batterysub-module (200 a), the second set of electronics (206 b) is powered bythe second battery sub-module (200 b), and the M^(th) set of electronics(206 c) is powered by the M^(th) battery sub-module (200 c). For brevityand to preserve the readability of the figure, voltage converters (e.g.,which step down the voltage produced by battery sub-modules to a voltagelevel that is expected by the electronics) are not shown herein but maybe used as or if needed.

The electronics (206 a-206 c) in this example include battery managementsystems (BMS) which monitor and/or record metrics and/or measurementsassociated with the cells within the associated battery sub-module overtime. In some embodiments, the battery management systems monitor and/ortrack the voltages of each of the cells in the associated batterysub-modules over time. The electronics controller (208) controls thevarious electronics (206 a-206 c) in ways described in more detailbelow.

This type of battery arrangement may be better suited for aircraftapplications compared to car applications. For example, the FederalAviation Administration may have very stringent requirements when itcomes to redundancy and/or potential single points of failures. Byarranging multiple battery sub-modules in series with backup connectionsnot shown, the overall battery system can still work and output ahigh-voltage signal for the high-voltage load (202) even if one of thebattery sub-modules fails. In contrast, the National Highway TrafficSafety Administration may not care as much about redundancy and/orpotential single points of failures because if the battery fails, thecar can just coast and pull over to the shoulder whereas an aircraftwould crash. For these and other reasons, battery systems for electriccars tend to be more monolithic (e.g., with relatively few batterysub-modules and/or relatively few cells per battery sub-module comparedto battery systems for aircrafts).

Due to slight differences between the various cells and various batterysub-modules, the voltages across the cells and battery sub-modules arenot all the same. Furthermore, due to the configuration shown here,battery sub-modules which have less charge will be used to supply powermore than battery sub-modules which have more charge (e.g., ifinter-module balancing, one example of which is described in FIG. 1, isnot performed) while the high-voltage load (202) is off and theelectronics (206 a-206 c) are on (e.g., when the aircraft is powereddown). To use an analogy, the rich (sub-modules) stay rich and the poor(sub-modules) stay poor. To address this, electronics controller 208(e.g., including a BMS controller) performs the balancing process ofFIG. 1.

In the context of this example system, step 100 of FIG. 1 is initiatedwhen the electronics controller (208) decides to perform the process ofFIG. 1. As described above, balancing may be performed before and/orafter charging but (e.g., for simplicity and/or to avoid expensivesampling equipment) balancing is not performed when the high-voltageload is drawing power from the battery system.

Once the process of FIG. 1 begins, the electronics controller (208)sends a signal to each set of electronics (206 a-206 c) to send back oneor more voltages associated with a cell in the associated batterysub-module. For example, the voltage sent back to the electronicscontroller may be the minimum (e.g., lowest) voltage of all of the cellsin that battery sub-module, sometimes referred to herein as the minimumcell voltage (e.g., for a given battery sub-module). In some otherembodiments, some other type of cell voltage (e.g., as a maximum cellvoltage or a median or mean cell voltage) is/are sent to the electronicscontroller in addition to and/or in place of a minimum cell voltage.Using the voltages received from the electronics (206 a-206 c), theelectronics controller selects at least one set of electronics. In oneexample, the electronics with the global minimum cell voltage (e.g., thecontroller picks the minimum of the minimum cell voltages) are selected(e.g., because continuing to draw power from that battery sub-module maypermanently damage the battery sub-module if the cell with the minimumcell voltage goes below some threshold and/or unrecoverable cell voltagelevel). This is one example of step 102 in FIG. 1.

In this example, there are two paths between each set of electronics(206 a-206 c) and the electronics controller (208). One path is forcommunications and/or or control and the other path is for power. Thelatter incorporates a switch to interrupt power to the electronicscontroller from a given battery sub-module and/or set of electronics.The control and/or communications path is always connected and available(e.g., to allow the controller to interrogate cell voltages and tocontrol the state of the aforementioned switch in response to voltagemeasurements).

The electronics controller then configures the selected electronics sothat it does not draw power (e.g., to the degree possible since there istypically some level of vampire power consumption even if things are“turned off”) from its associated battery sub-module. In someembodiments, the electronic controller turns off the selectedelectronics to achieve this goal. Alternatively, the electroniccontroller in some other embodiments configures the selected electronics(and/or any other components) so that power from a given batterysub-module is not sent upstream to the electronics controller (208). Forexample, even if electronics 206 a is in power minimization mode and notproviding any power to the controller (208), the controller (208) canstill interrogate battery management system 206 a for its voltages, etc.This may be desirable in applications where it is desirable to keep theelectronics accessible. For example, as described above, a batterymanagement system tracks and/or monitors metrics associated with theassociated battery sub-module and/or the cells within. It may bedesirable to keep tracking such metrics and/or measurements, for exampleby obtaining power from another battery sub-module. These are someexamples of how step 104 in FIG. 1 may be performed.

Without balancing, one or more of the battery sub-modules may beirreparably damaged within as soon as ˜20 days. For example, if thevoltage level of a cell drops below some voltage level and powercontinues to be drawn from that cell, the cell will be irreparablydamaged and as a result the entire battery sub-module will need to bereplaced.

The following figures describe some of the examples described above moregenerally and/or formally in flowcharts.

FIG. 4 is a flowchart illustrating an embodiment of a process forinter-module balancing, including by turning off electronics. FIG. 4 isrelated to FIG. 1 and, for convenience, related steps are indicatedusing similar or the same reference numbers.

At 100, for each battery sub-module in a plurality of batterysub-modules, a voltage associated with a cell in that battery sub-moduleis received, wherein each battery sub-module in the plurality of batterysub-modules includes a plurality of cells. For example, the electronicscontroller (208) in FIG. 2 receives at least one voltage from each ofelectronics (206 a-206 c) where each received voltage is associated witha cell in the corresponding or associated battery sub-module (200 a-200c).

At 102, a battery sub-module is selected from the plurality of batterysub-modules based at least in part on the received voltages. Someexamples of how the selection may be performed are described in moredetail below. In some embodiments, multiple battery sub-modules areselected.

At 104 a, a set of one or more loads, which draws power from theselected battery sub-module and is not powered by any other batterysub-module in the plurality of battery sub-modules, is configured sothat the set of one or more loads at least temporarily does not drawpower from the selected battery sub-module, including by configuring theset of loads which draws power from the selected battery sub-module tobe off. For example, if the first battery sub-module (200 a) in FIG. 2is selected, then loads controller 208 may configure the first set ofloads 206 a so that they are off and do not draw power from the firstbattery sub-module (200 a).

FIG. 5 is a flowchart illustrating an embodiment of a process forinter-module balancing, including by configuring a set of electronics todraw power from an unselected battery sub-module. FIG. 5 is related toFIG. 1 and, for convenience, related steps are indicated using similaror the same reference numbers.

At 100, for each battery sub-module in a plurality of batterysub-modules, a voltage associated with a cell in that battery sub-moduleis received, wherein each battery sub-module in the plurality of batterysub-modules includes a plurality of cells.

At 102, a battery sub-module is selected from the plurality of batterysub-modules based at least in part on the received voltages. Asdescribed above, in some embodiments, multiple battery sub-modules areselected (e.g., because multiple battery sub-modules are in a poor stateto supply power and/or may be irreparably damaged if they continue tosupply power and are thus selected).

At 104 b, a set of one or more loads, which draws power from theselected battery sub-module and is not powered by any other batterysub-module in the plurality of battery sub-modules, is configured sothat the set of one or more loads at least temporarily does not drawpower from the selected battery sub-module, including by configuring theset of loads which draws power from the selected battery sub-module todraw power from an unselected battery sub-module.

In some applications, it is undesirable to turn off the electronics. Inthe example of FIG. 2, the electronics include battery managementsystems which track and/or monitor the health and/or other metrics ofthe battery sub-modules and/or cells and it is important and/ordesirable to track that information at all times. For example, with anaircraft, the aircraft may be powered down on weekdays and only flown onthe weekend. The battery management systems should run throughout theweek so that any bad battery sub-modules can be identified and/or anaircraft is not permitted to fly as or if needed.

As described above, in some embodiments, a battery sub-module isselected from the plurality of battery sub-modules in order to preventcells in that battery sub-module from being drawn down to a voltagelevel at which irreparable damage occurs (e.g., and the entire batterysub-module must be replaced). The following figures describe someexemplary cell voltages and exemplary techniques for selecting a batterysub-module using those cell voltages.

FIG. 6 is a diagram illustrating an embodiment of cell voltages inbattery sub-modules in a battery system. In this example, there are Mbattery sub-modules and N cells per battery sub-modules to be consistentwith the example of FIG. 2. In the graph shown, the x-axis shows thecell index (defined by battery sub-module number and cell number withinthat battery sub-module) and the y-axis shows the cell voltage of thecorresponding cell. Group 600 shows the cell voltages for the cells inthe first battery sub-module, group 602 shows the cell voltages for thecells in the second battery sub-module, and group 604 shows the cellvoltages for the cells in the M^(th) battery sub-module.

For simplicity and ease of explanation, suppose that there is a voltagelevel, represented by V_(threshold) (606), below which a cell will bepermanently damaged if power continues to be drawn from that cell (e.g.,standby or vampire power when the aircraft is powered down). Forexample, cell 2,1 (610) and cell 2,N (612), both of which are in thesecond battery sub-module (602), are at or below V_(threshold) (606). Toensure that power is not further drawn from that battery sub-module, thesecond battery sub-module would be selected (e.g., at step 102 inFIG. 1) and the corresponding set of electronics (e.g., 206 b in FIG. 2)would be configured so that they no longer draw power from the secondbattery sub-module (e.g., 200 b in FIG. 2).

Returning briefly to FIG. 2, it would be desirable if the electronicscontroller (208) could receive cell voltages for only some cells perbattery sub-modules instead of having to receive cell voltages for allcells in a given battery sub-module. This would, for example, reduce theamount of traffic or communications exchanged between the electronicscontroller (208) and the lower-level electronics (206 a-206 c).

In one example, the minimum cell voltage from each battery sub-module issent to an electronics controller or other block which is making theselection. For example, the respective electronics controller (e.g.,battery management system) may make this selection and upload only theminimum cell voltage to the electronics controller (e.g., BMScontroller). In FIG. 6, this would mean selecting and sending the cellvoltage for cell 1,(N−1) (620), which is the minimum cell voltage in thefirst battery sub-module (600), to such an electronics controller. Forthe second battery sub-module (602), the minimum cell voltage is thecell voltage for cell 2,1 (610) and would be selected and sent to theelectronics controller. For the M^(th) battery sub-module (604), theminimum cell voltage is the cell voltage for cell M,N (622) and thatcell voltage would be selected and sent to the electronics controller.

In some embodiments, a battery sub-module is selected at step 102 inFIG. 1 using a threshold voltage. For example, if any battery sub-modulehas a minimum cell voltage that is below V_(threshold) (606), then thatbattery sub-module is selected so that its corresponding electronics donot continue to draw power from that battery sub-module. For theexemplary cell voltages shown in FIG. 6, only the second batterysub-module (602) would be selected. As such, the correspondingelectronics would be configured to at least temporarily not draw powerfrom the second battery sub-module (e.g., either by turning off thesecond set of electronics or providing power from some other batterysub-module).

In some embodiments, the above steps are performed first (e.g., wherethe minimum cell voltage from each battery sub-module is comparedagainst some voltage threshold, such as V_(threshold) (606)). Then, fromthe pool of battery sub-modules which were above V_(threshold), the nbattery sub-modules with the n maximums of the (remaining) minimum cellvoltages are used to provide power (at least temporarily) with the otherbattery cell-modules (including those with a minimum cell voltage belowV_(threshold)) not providing power (at least temporarily). This drawsdown the n battery sub-modules, which makes those battery sub-modulesmore balanced with respect to the other battery sub-modules.

In FIG. 6, for example, the M^(th) battery sub-module (604) has muchhigher cell voltages in general compared to the rest of the batterysub-modules. By drawing down the cell voltages of the M^(th) batterysub-module (604), this may help to draw down the M^(th) batterysub-module (604), the high-end outlier, without irreparably damaging thesecond battery sub-module (602), the low-end outlier. To put it anotherway, the first check or test (e.g., comparing the minimum cell voltagesagainst V_(threshold)) ensures nothing fails or is broken and the secondcheck or test (e.g., drawing power from the n battery sub-modules withthe n maximums of the minimum cell voltages) is a performance-orientedselection (e.g., it does a better job at balancing than some otherselection techniques and balanced battery sub-modules is good forperformance).

In some embodiments, there are no battery sub-modules with minimum cellvoltages below V_(threshold) (606). In some such embodiments, themaximum cell voltage from each battery sub-module is obtained and the mbattery sub-modules with the m maximums of the maximum cell voltagesprovide power (at least temporarily) while the rest of the batterysub-modules do not provide power (at least temporarily). In thissituation, there is no battery sub-module which is in danger of beingpermanently damaged if it continues to supply power and so using themaximum cell voltage from each cell is an even better way to balance thesub-modules (e.g., even better than using the maximum of the minimumcell voltages). In the context of this kind of balancing, it's alwaysbeneficial to draw power from higher-voltage sub-modules thanlower-voltage ones.

These examples are described more generally and/or formally inflowcharts below. In various applications and/or embodiments, theappropriate technique may be performed.

FIG. 7 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold. In some embodiments, abattery sub-module is selected at step 102 in FIG. 1 using the exampleprocess described herein. In this example, receiving voltages at step100 in FIG. 1 includes receiving, for each battery sub-module in theplurality of battery sub-modules connected together in series, a minimumcell voltage such that a plurality of minimum cell voltages is received.

At 700, the plurality of minimum cell voltages is compared against avoltage threshold in order to identify any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold. Forexample, in FIG. 6, the minimum cell voltages for cell 1,(N−1) (620),cell 2,1 (610), and cell M,N (622) are compared against V_(threshold)(606). In that example, the only cell with a minimum cell voltage thatdoes not exceed the voltage threshold is cell 2,1 (610).

At 702, any said identified battery sub-modules with a minimum cellvoltage that does not exceed the voltage threshold is selected. Tocontinue the example from FIG. 6, the second battery sub-module (602)would be selected. As such, the corresponding electronics would beconfigured so they do not draw power from the second battery sub-module(602), at least temporarily. The second battery sub-module (602) isvulnerable and could be damaged permanently if it continues to drawpower.

Depending upon the design objectives and/or constraints, the appropriatetechnique for making a selection may be used. For example, the processof FIG. 7 is relatively simple. In some applications, if the performanceimprovements offered by other, more complicated processes is/are onlymarginal, then the process of FIG. 7 is used to make a selection.

FIG. 8 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold and maximums of minimumcell voltages. In some embodiments, a battery sub-module is selected atstep 102 in FIG. 1 using the example process described herein. In thisexample, receiving voltages at step 100 in FIG. 1 includes receiving,for each battery sub-module in the plurality of battery sub-modulesconnected together in series, a minimum cell voltage such that aplurality of minimum cell voltages is received.

At 800, the plurality of minimum cell voltages is compared against avoltage threshold in order to identify any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold. See,for example, FIG. 6 where the second battery sub-module (602) has aminimum cell voltage (610) that does not exceed the voltage threshold(606).

At 802, one or more maximums are selected from the plurality of minimumcell voltages in order to obtain one or more maximums of the minimumcell voltages. For example, in FIG. 6, the minimum cell voltages includethe voltages for cell 1,(N−1) (620), cell 2,1 (610), and cell M,N (622)and the maximum of those is the voltage for cell M,N (622). Forsimplicity and ease of explanation, suppose that only one maximum isselected in this example of step 802 and subsequently at step 804.

At 804, said any identified battery sub-modules with a minimum cellvoltage that does not exceed the voltage threshold is selected, as wellas those battery sub-modules that do not correspond to one of themaximums of the minimum cell voltages. For example, the second batterysub-module (602) would be selected because it has a minimum cell voltage(610) that does not exceed the voltage threshold (606). Also, the firstbattery sub-module (600) does not correspond to the maximum of theminimum cell voltages and so the first battery sub-module would also beselected. In other words, the first battery sub-module (600) and thesecond battery sub-module (602) would not have to provide power (atleast temporarily) while the M^(th) battery sub-module (604) wouldprovide power (e.g., during the time period in question). Intuitively,this makes sense because the M^(th) battery sub-module (604) tends tohave higher cell voltages compared to the other battery sub-modules.

In some applications, the process of FIG. 8 is used instead of theprocess of FIG. 7 because it enables better and/or faster balancingcompared to FIG. 7, but without having to obtain additional cellvoltages for each battery sub-module (e.g., per FIG. 9).

FIG. 9 is a flowchart illustrating an embodiment of a process to selecta battery sub-module using a voltage threshold and maximums of maximumcell voltages. In some embodiments, a battery sub-module is selected atstep 102 in FIG. 1 using the example process described herein. In thisexample, receiving voltages at step 100 in FIG. 1 includes receiving,for each battery sub-module in the plurality of battery sub-modulesconnected together in series, a minimum cell voltage and a maximum cellvoltage such that a plurality of minimum cell voltages and a pluralityof maximum cell voltages are received.

At 900, the plurality of minimum cell voltages is compared against avoltage threshold in order to identify any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold. See,for example, FIG. 6.

At 902, it is determined if there are any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold. Forexample, with the cell voltages shown in FIG. 6, this decision would be“Yes” because the minimum cell voltage for cell 2,1 (610) does notexceed the voltage threshold (606). In this example, the process wouldthen proceed to step 802 in FIG. 8.

If, however, the decision at step 902 is “No” (e.g., because all of theminimum cell voltages exceed the voltage threshold), then one or moremaximums are selected from the plurality of maximum cell voltages inorder to obtain one or more maximums of the maximum cell voltages at904. For example, the plurality of maximum cell voltages in FIG. 6includes the cell voltages for cell 1,2 (630), cell 2,(N−1) (632), andcell M,1 (634). If only one maximum is selected, then the maximum of themaximum cell voltages would be the cell voltage for cell M,1 (634).

At 906, those battery sub-modules that do not correspond to one of themaximums of the maximum cell voltages are selected. In other words, thebattery sub-modules corresponding to the maximums of the maximum cellvoltages will provide power (at least temporarily) for some period oftime. To continue the example from above, the M^(th) battery sub-modulewould provide power (at least temporarily) while the other batterysub-modules would not provide power (at least temporarily).

In some applications, this technique enables the best and/or fasterbalancing but requires the use of both minimum cell voltages and maximumcell voltages, which requires more information to be exchanged betweenthe (local) electronics (e.g., battery management systems) and theelectronics controller (e.g., BMS controller). Depending upon theparticular design objectives and/or limitations of the particularapplication, an appropriate technique may be selected. For example, ifperformance is important and the exchange of more and/or additional isan acceptable trade-off, then the process of FIG. 9 may be used.

As described above, in some embodiments, balancing is performed beforeand/or after charging. The following figures describe some examplescenarios where balancing is performed both before and after charging,as well as only after charging.

FIG. 10A is a diagram illustrating an embodiment where balancing isperformed both before and after charging. In the example shown, thebattery system has major imbalances between the various batterysub-modules and/or their underlying cells at time 0. For example,suppose that a BMS controller calculates an imbalance metricrepresenting a degree or amount of imbalance in the battery system andthe metric is relatively high and/or above some imbalance threshold. Asdescribed above, it is important for the battery system to be (e.g.,sufficiently) balanced before charging. As such, in this example, afirst pass of balancing is performed at 1000 (e.g., per any of thebalancing techniques described above). For example, some batterysub-modules will provide power to various electronics in the systemwhile other battery sub-modules do not provide power for some predefinedamount of time and/or until some desired imbalance metric is reached.

Then, after the first pass or iteration of balancing is performed at1000, the battery system is charged at 1002.

After charging (1002) has completed, there may still be some degree ofimbalance in the battery system (e.g., carried over from the end of thefirst balancing pass) and/or additional imbalances may have beenintroduced by the charging process. As such, a second pass or iterationof balancing (e.g., per any of the techniques described above) isperformed at 1004, but this time to address smaller and/or minorimbalances in the battery system.

FIG. 10B is a diagram illustrating an embodiment where balancing isperformed only after charging. In this example, the battery system has arelatively small amount or degree of imbalance between the batterysub-modules (and/or the underlying cells) when the overall processbegins. To put it another way, at time 0, the battery sub-modules aresufficiently balanced so that charging can be performed immediately(e.g., without first having to run a balancing process). As before, aBMS controller may have determined an imbalance metric and compared itagainst some threshold in order to conclude that the battery system issufficiently balanced to proceed with charging. As such, charging (1050)is immediately performed at time=0. After charging completes, balancing(1052) is performed (e.g., per any of the techniques described above) toaddress relatively small and/or minor imbalances existing in the batterysystem at that time.

A third possible scenario (not shown here for brevity) is to performbalancing before charging, but not after charging.

The following figure describes the above examples more generally and/orformally in a flowchart.

FIG. 11 is a flowchart illustrating an embodiment of a process to decidewhen to perform balancing relative to a charging process. In someembodiments, the process is performed by BMS controller 208 in FIG. 2.

At 1100, an imbalance metric associated with a degree of imbalancebetween battery sub-modules in the plurality of battery sub-modules isdetermined. An example of an imbalance metric is a difference betweenthe maximum cell state of charge and minimum cell state of within abattery, referred to herein as RANGE(SOC). Another metric in thisexample is the amount of imbalance that can be dealt with over(?) theduration of one charge, referred to herein as maxImbalance.Straightforwardly, if RANGE(SOC)>maxImbalance, then it would bebeneficial to balance before charging. If balance were not done beforecharging, then the battery would be charged until the maximum voltagecell reached the maximum cell voltage threshold (where going above thisthreshold would damage the cell). At this point, the battery would stillbe imbalanced and all high voltage cells would need to be drained untilthey reached the same voltage as the minimum voltage cell. After this,another charge would be performed until the now-balanced battery reachedthe max cell voltage.

This isn't really an issue if the aircraft is left attached to a chargerfor a very long period of time. In this case, the battery could betrickle-charged and kept topped off as the battery balances itself.However, in a high throughput environment where aircraft need to spendminimal time on the charger (e.g., an air taxi or shared useapplication), it is beneficial to pre-balance the battery (e.g., sincethere is no need to have a battery connected to a charger during thattime).

Note that maxImbalance is actually a variable and not a fixed value. Ifthe aircraft is fully discharged, it can nominally take 1.25 hours tocharge. Since balancing can occur while charging and balancing happensat a set rate, the logic follows that if there is less than 1.25 hours'worth of balancing required, charging should proceed or otherwise bedone without any fear of downtime. Otherwise, there would be a benefitfrom balancing beforehand if it is desired to minimize time on thecharger. If only 0.5 hours' worth of charge is required (i.e., the planewas only partially discharged), then the threshold gets correspondinglysmaller.

At 1102, it is determined if the imbalance metric exceeds an imbalancethreshold. In this example, an imbalance metric with a larger valuecorresponds to a larger degree or amount of imbalance in the batterysystem and an imbalance metric with a smaller value corresponds to asmaller degree or amount of imbalance in the battery system. To put itanother way, the imbalance threshold is used to decide if the batterysystem is sufficiently charged to begin charging right away, or if somebalancing needs to be performed first.

If the imbalance metric exceeds the imbalance threshold at 1102 (e.g.,the battery system is not sufficiently balanced for charging), thenpre-charging balancing is performed at 1104. For example, any of thebalance techniques described above (e.g., FIG. 1) may be used. Afterpre-charging balancing is performed at 1104, the plurality of batterysub-modules are charged at 1106.

If the imbalance metric does not exceed the imbalance threshold at 1102(e.g., the battery system is sufficiently balanced for charging), thenthe plurality of battery sub-modules are charged at 1106 (e.g., withoutfirst performing balancing at step 1104).

In some embodiments, after the battery sub-modules are charged at step1106, post-charging balancing is performed at 1108 (e.g., using any ofthe above described balancing techniques, such as FIG. 1).Alternatively, the step of post-charging balancing at 1108 may beskipped (e.g., because the degree or amount of imbalance in the batterysystem after charging does not warrant an iteration of balancing).

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a plurality of batterysub-modules, wherein: the plurality of battery sub-modules areelectrically connected in series in order to provide power to a primaryload; each battery sub-module includes a plurality of cells electricallyconnected in series; and each battery sub-module further includes abattery management system that monitors the plurality of cells in thatbattery sub-module; and to a controller that is configured to: selectone or more battery sub-modules from the plurality of batterysub-modules, including by: obtaining at least one voltage from eachbattery sub-module; and selecting one or more battery sub-modules basedat least in part on the obtained voltages; and turn off those batterymanagement systems in the one or more selected battery sub-modules sothat the battery management systems in the one or more selected batterysub-modules do not consume power at least temporarily from the pluralityof cells in the one or more selected battery sub-modules while (1) theplurality of battery sub-modules is not providing power to the primaryload and (2) the plurality of battery sub-modules is not being charged.2. The system recited in claim 1, wherein selecting the one or morebattery sub-modules is based at least in part on a plurality of minimumcell voltages which includes a minimum cell voltage from each batterysub-module in the plurality of battery sub-modules.
 3. The systemrecited in claim 1, wherein selecting the one or more batterysub-modules is based at least in part on a plurality of maximum cellvoltages which includes a maximum cell voltage from each batterysub-module in the plurality of battery sub-modules.
 4. The systemrecited in claim 1, wherein selecting the one or more batterysub-modules includes: comparing, for each battery sub-module in theplurality of battery sub-modules, a minimum cell voltage against avoltage threshold in order to identify any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold; andselecting one or more of those battery sub-modules with a minimum cellvoltage that does not exceed the voltage threshold.
 5. The systemrecited in claim 1, wherein the controller is further configured to:determine an imbalance metric associated with a degree of imbalancebetween battery sub-modules in the plurality of battery sub-modules;determine whether the imbalance metric exceeds an imbalance threshold;and in response to determining that the imbalance metric exceeds theimbalance threshold: performing pre-charging balancing, including by:selecting one or more battery sub-modules from the plurality of batterysub-modules, including by: obtaining at least one voltage from eachbattery sub-module; and selecting one or more battery sub-modules basedat least in part on the obtained voltages; and turning off those batterymanagement systems in the one or more selected battery sub-modules sothat the battery management systems in the one or more selected batterysub-modules do not consume power at least temporarily from the pluralityof cells in the one or more selected battery sub-modules while (1) theplurality of battery sub-modules is not providing power to the primaryload and (2) the plurality of battery sub-modules is not being charged;and after performing pre-charging balancing, charging the plurality ofbattery sub-modules.
 6. A method, comprising: selecting one or morebattery sub-modules from a plurality of battery sub-modules, includingby: obtaining at least one voltage from each battery sub-module; andselecting one or more battery sub-modules based at least in part on theobtained voltages, wherein: the plurality of battery sub-modules areelectrically connected in series in order to provide power to a primaryload; each battery sub-module includes a plurality of cells electricallyconnected in series; and each battery sub-module further includes abattery management system that monitors the plurality of cells in thatbattery sub-module; and turning off those battery management systems inthe one or more selected battery sub-modules so that the batterymanagement systems in the one or more selected battery sub-modules donot consume power at least temporarily from the plurality of cells inthe one or more selected battery sub-modules while (1) the plurality ofbattery sub-modules is not providing power to the primary load and (2)the plurality of battery sub-modules is not being charged.
 7. The methodrecited in claim 6, wherein selecting the one or more batterysub-modules is based at least in part on a plurality of minimum cellvoltages which includes a minimum cell voltage from each batterysub-module in the plurality of battery sub-modules.
 8. The methodrecited in claim 6, wherein selecting the one or more batterysub-modules is based at least in part on a plurality of maximum cellvoltages which includes a maximum cell voltage from each batterysub-module in the plurality of battery sub-modules.
 9. The methodrecited in claim 6, wherein selecting the one or more batterysub-modules includes: comparing, for each battery sub-module in theplurality of battery sub-modules, a minimum cell voltage against avoltage threshold in order to identify any battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold; andselecting one or more of those battery sub-modules with a minimum cellvoltage that does not exceed the voltage threshold.
 10. The methodrecited in claim 6, further comprising: determining an imbalance metricassociated with a degree of imbalance between battery sub-modules in theplurality of battery sub-modules; determining whether the imbalancemetric exceeds an imbalance threshold; and in response to determiningthat the imbalance metric exceeds the imbalance threshold: performingpre-charging balancing, including by: selecting one or more batterysub-modules from the plurality of battery sub-modules, including by:obtaining at least one voltage from each battery sub-module; andselecting one or more battery sub-modules based at least in part on theobtained voltages; and turning off those battery management systems inthe one or more selected battery sub-modules so that the batterymanagement systems in the one or more selected battery sub-modules donot consume power at least temporarily from the plurality of cells inthe one or more selected battery sub-modules while (1) the plurality ofbattery sub-modules is not providing power to the primary load and (2)the plurality of battery sub-modules is not being charged; and afterperforming pre-charging balancing, charging the plurality of batterysub-modules.
 11. A computer program product, the computer programproduct being embodied in a non-transitory computer readable storagemedium and comprising computer instructions for: selecting one or morebattery sub-modules from a plurality of battery sub-modules, includingby: obtaining at least one voltage from each battery sub-module; andselecting one or more battery sub-modules based at least in part on theobtained voltages, wherein: the plurality of battery sub-modules areelectrically connected in series in order to provide power to a primaryload; each battery sub-module includes a plurality of cells electricallyconnected in series; and each battery sub-module further includes abattery management system that monitors the plurality of cells in thatbattery sub-module; and turning off those battery management systems inthe one or more selected battery sub-modules so that the batterymanagement systems in the one or more selected battery sub-modules donot consume power at least temporarily from the plurality of cells inthe one or more selected battery sub-modules while (1) the plurality ofbattery sub-modules is not providing power to the primary load and (2)the plurality of battery sub-modules is not being charged.
 12. Thecomputer program product recited in claim 11, wherein selecting the oneor more battery sub-modules is based at least in part on a plurality ofminimum cell voltages which includes a minimum cell voltage from eachbattery sub-module in the plurality of battery sub-modules.
 13. Thecomputer program product recited in claim 11, wherein selecting the oneor more battery sub-modules is based at least in part on a plurality ofmaximum cell voltages which includes a maximum cell voltage from eachbattery sub-module in the plurality of battery sub-modules.
 14. Thecomputer program product recited in claim 11, wherein selecting the oneor more battery sub-modules includes: comparing, for each batterysub-module in the plurality of battery sub-modules, a minimum cellvoltage against a voltage threshold in order to identify any batterysub-modules with a minimum cell voltage that does not exceed the voltagethreshold; and selecting one or more of those battery sub-modules with aminimum cell voltage that does not exceed the voltage threshold.
 15. Thecomputer program product recited in claim 11, further comprisingcomputer instructions for: determining an imbalance metric associatedwith a degree of imbalance between battery sub-modules in the pluralityof battery sub-modules; determining whether the imbalance metric exceedsan imbalance threshold; and in response to determining that theimbalance metric exceeds the imbalance threshold: performingpre-charging balancing, including by: selecting one or more batterysub-modules from the plurality of battery sub-modules, including by:obtaining at least one voltage from each battery sub-module; andselecting one or more battery sub-modules based at least in part on theobtained voltages; and turning off those battery management systems inthe one or more selected battery sub-modules so that the batterymanagement systems in the one or more selected battery sub-modules donot consume power at least temporarily from the plurality of cells inthe one or more selected battery sub-modules while (1) the plurality ofbattery sub-modules is not providing power to the primary load and (2)the plurality of battery sub-modules is not being charged; and afterperforming pre-charging balancing, charging the plurality of batterysub-modules.